## **Core Concept**
The question tests knowledge of common causes of unilateral parotid swelling, particularly in a young adult. Parotid swelling can result from various pathologies, including infections, neoplasms, and obstructive disorders. The differential diagnosis varies based on the patient's age, symptoms, and specific characteristics of the swelling.

## **Why the Correct Answer is Right**
The correct answer, **Pleomorphic adenoma (Benign mixed tumor)**, is the most common cause of unilateral parotid swelling in a young adult like the 27-year-old male in the question. Pleomorphic adenomas are benign neoplasms that arise from the salivary gland tissue, particularly the parotid gland. They are known for their slow growth and are often found in the superficial lobe of the parotid gland. Their etiology is not entirely clear, but they are believed to arise from the ductal or myoepithelial components of the salivary gland.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While **Parotid abscess** can cause unilateral parotid swelling, it is typically accompanied by signs of acute infection such as pain, redness, swelling, and sometimes fever. It is less likely to be the most common cause in the absence of specific mention of infectious symptoms.
- **Option B:** **Parotid duct obstruction** can cause swelling but usually presents with a history of sialolithiasis (salivary gland stones) or other obstructive causes. It's less common as a primary presentation compared to a neoplastic cause like pleomorphic adenoma.
- **Option C:** This option is not provided, so we proceed with the understanding that the correct answer is linked to a specific pathology not listed here.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that when evaluating unilateral parotid swelling, especially in a young adult, a neoplastic cause like **pleomorphic adenoma** should be considered high in the differential diagnosis. Although the majority of parotid tumors are benign, the possibility of malignancy necessitates proper evaluation and potentially biopsy.

## **Correct Answer: D. Pleomorphic adenoma.**